The present invention relates to an automatic power factor control apparatus, and in particular to an automatic power factor control apparatus for an inductive load such as an AC induction motor.
Proposals have been made in the prior art for automatically improving the power factor of an inductive load such as an AC induction motor, such as in U.S. Pat. Nos. 4,052,648 and 4,337,640. In these proposals, the power factor is improved by means of phase control of an input voltage that is applied to drive an induction motor. With such types of automatic power factor control apparatus, a thyristor is connected in the AC power supply line, and direct phase control is executed of the AC power voltage that is supplied to the inductive load. As a result, the load current contains large amounts of electromagnetic noise and high frequency components, which results in considerable interference with data processing apparatus such as computers, and with other types of control apparatus such as communication controllers etc. Moreover, the thyristor is triggered once in every cycle of the AC power supply, synchronized with that power supply voltage. However since the phase signal that is used to trigger the thyristor is obtained from the power supply voltage, that signal will be affected by variations in the waveform of that power supply voltage. For that reason, control is not stable, but is affected by the load conditions, and in some conditions control will become impossible. It is therefore difficult to produce an automatic power factor control apparatus that will provide stable operation and reliability, with such prior art methods.
In an attempt to overcome these problems, a proposal has been made for providing a high frequency filter in such an apparatus, in U.S. Pat. No. 4,602,200. However with that proposed apparatus, it is necessary to use a number of high-value capacitors as well as reactors and resistors, so that the overall size of the apparatus would be large and the manufacturing cost would be high. Furthermore, semiconductor elements for electrical power applications have a relatively low degree of overload withstanding capability, so that destruction of circuit elements can readily occur when current surges are produced at the time of start-up of operation of an inductive load. This can result in frequent stoppages of the apparatus for repairs and maintenance. In order to prevent this, it is necessary to use semiconductor elements having a large power-handling capacity, and to use high levels of power for control. This is uneconomical, and electrical losses are high. In addition, it is necessary to provide large heat-dissipation fins on some circuit elements, so that such an automatic power factor control apparatus would present problems with regard to attaining a compact size and low manufacturing cost.